Measurement cells of the kind and methods for analysing samples by means of electrochemiluminescence tests, particularly immunoassay tests using such measurement cells, are, for example known from DE 43 42 942 A1, DE 198 03 528 A1, WO89/10551 A1 and WO90/11511 so that for the understanding of the basic technology concerning the subject-matter of the present invention reference is made to these publications.
When analysing a liquid sample by means of electrochemiluminescence tests usually the concentration of a substance (analyte) contained in the sample liquid is to be determined. In the medical field particularly the analysis of body fluids like blood, urine, saliva etc. is of great importance, in view of analytes contained therein, as for example antibodies, antigens, hormones etc.
A typical measurement process in such tests comprises the multiple exchange of liquids and/or mixtures in the measurement cell. Hence, during a typical measurement, a first mixture is induced into the cleaned measurement cell through the fluid inlet channel into the measurement cell cavity. The first mixture is an incubate of the sample, reagents and magnetic particles. In the present considered tests the complex-molecules, which are marked with an electrochemiluminescence marker substance and are characteristic for the analysis, are fixed to these magnetic particles. Such a fixation is effected by a pair of specific biochemical binding partners, whereby particularly the pair streptavidin-biotin proved of value. The magnetic particles are for example coated with streptavidin-polymer, whereas biotin is bound to the complex-molecules.
In known measurement cells the magnetic particles are trapped to the surface of the working electrode together with the marked complex bound thereto in the magnetic field of a magnet arranged close to the working electrode. This may be effected during the continuous flow of the first mixture, whereby incubation fluid discharges from the measurement cell cavity through the fluid outlet channel. The accumulation of the magnetic particles on the working electrodes while discharging incubation fluid is called bound free separation.
After trapping the magnetic particles, a measurement reagent may be induced into the cell in a next step, whereby the magnetic particles are washed by this measurement reagent. This step of washing is to remove unbound components from the working electrode which potentially interfere with the electrochemical reaction.
Thereafter the electrochemiluminescence reaction is triggered by application of an electric potential to the working electrode, whereby the intensity of the luminescence light is detected by means of a photosensor and may be evaluated as a measure for the concentration of the marked magnetic particles on the surface of the working electrode, whereby this concentration again serves as a measure for the concentration of the analyte in the sample.
After the electrochemiluminescence measurement the cell usually is rinsed with a cleaning fluid, which in a further step may be discharged with the measurement reagent in order to condition the cell for the next measurement.
It is essential for the quality of the measurement that the above-mentioned washing step is efficient, so that in the mixture of measurement reagent and magnetic particles, separated from the incubate, the least possible amount of interfering components, as for example sample components, is contained. Such interfering components could cause changes of the measurement signal. Such measurement interferences are also called matrix effects. If the above-mentioned washing step is executed too violently, this may, however, also lead to negative effects, if—for example—due to too large flow velocities, turbulences, etc., magnetic particles are removed from their position on the working electrode.
In known measurement cells the fluid inlet channel and the fluid outlet channel meet the measurement cell cavity orthogonally to the longitudinal direction of the oblong measurement cell cavity, so that, when fluid is passed through the measurement cell, the respective fluid flow is abruptly deflected by an angle of 90° when being induced into the measurement cell cavity—and finally again by an angle of 90° when being discharged from the measurement cell cavity. Such a geometry of the fluid channels was established due to reasons of construction and production, and hitherto has been considered to be well suited for an optimal operation of the measurement cell.
In known measurement cells the housing thereof comprises a base block which is interspersed by the fluid inlet channel and the fluid outlet channel and delimits the measurement cell cavity by one of its lateral surfaces, with the working electrode being provided on the peripheral face of the measurement cell cavity. The fluid channels penetrate the base block and extend orthogonally with respect to the plane of the peripheral face of the measurement cell cavity of the base block. A spacer acting as a washer and having a central clearance is seated on the base block and forms the limit of the side wall of the measurement cell cavity with its internal contour. An acrylic glass panel is positioned on the spacer-washer as an optical window, on which the counter electrode is provided opposite to the working electrode.